1. Field of the Invention
The present invention relates to a water disintegratable sheet of which fibers can be dispersed in a large amount of water, more particularly, relates to a water disintegratable sheet which can offer a good balance of strength and water disintegratability.
2. Description of the Related Art
It is preferred that wet sheets for wiping a discharging part of a human body and wet wipers for cleaning a toilet are disintegratable in water. In absorbent articles such as sanitary napkin, panty liner and disposable diaper, it is also preferred that a topsheet covering a top surface of an absorbent layer and a backsheet covering a bottom surface of the absorbent layer are disintegratable in water. In addition, packaging sheets for packaging such absorbent articles are also preferably disintegratable in water.
If these sheets are disintegratable in water, they can be disposed of in a flush toilet after use. Such water disintegratable sheet disposed of in a flush toilet is immersed in a large amount of water in a flush toilet or a septic tank, and constituent fibers of the water disintegratable sheet are dispersed in water, thereby preventing the sheet from floating and remaining in a septic tank.
In such water disintegratable sheet, dry strength should be excellent as well as wet strength should be increased to some extent. When put in a large amount of water, on the other hand, the constituent fibers should be rapidly dispersed.
Japanese Unexamined Patent Publication No. H10-140494 (1998/140494) discloses a water disintegratable paper which is manufactured by impregnating a nonwoven fabric or paper with a pH reactive binder for increasing the strength and a pH buffer solution which is prepared with an organic acid to have an acidic pH. With the pH reactive binder, the water disintegratable paper has high strength when it has an acidic pH, so as not to disintegrate in water. When it has a neutral or alkaline pH, on the other hand, the paper is intended to disintegrate in water. In detail, when the water disintegratable paper is immersed in a large amount of water for neutralization, the binder is dissolved in water, so that fibers forming the water disintegratable paper are dispersed for disintegration of the water disintegratable paper in water.
On the other hand, Japanese Unexamined Patent Publication No. H5-279985 (1993/279985) discloses a nonwoven sheet formed only of ramie cellulose fibers. This nonwoven sheet comprises fibrillated ramie fibers and microfibrillated ramie fibers. The microfibrillated ramie fibers function as a binder for bonding the fibrillated ramie fibers to obtain sheet strength.
However, since the water disintegratable paper disclosed in Japanese Unexamined Patent Publication No. H10-140494 is impregnated with the pH reactive binder and the pH buffer solution, it may possibly exert a baneful influence upon the body of a user. In addition, when the water disintegratable paper is disposed of in natural environment, the pH reactive binder and the pH buffer solution added to the water disintegratable paper may possibly exert a baneful influence upon natural environment. Moreover, the organic acid contained in the pH buffer solution may possibly change with passage of time, which may possibly have an adverse effect upon the properties of the water disintegratable paper. Still moreover, the water disintegratable paper impregnated with the pH reactive binder is inferior in softness, so that a user cannot use it comfortably. Still moreover, since the pH reactive binder is expensive, the water disintegratable paper impregnated with the pH reactive binder cannot be manufactured at a low cost. Commonly, a water disintegratable sheet is impregnated with a solution of chemicals such as humectant, anti-inflammatory agent, anti-bacterial agent, surfactant, alcohol and perfume, depending on the purpose of the usage. However, if the sheet is impregnated with an inhibitor for inhibiting the pH reactive binder from dissolving, the chemicals must be ones that not react with the inhibitor, so that the selection of chemicals is severely limited.
On the other hand, since the nonwoven sheet disclosed in Japanese Unexamined Patent Publication No. H5-279985 is formed only of the ramie fibers, the ramie fibers are strongly hydrogen bonded to each other. Therefore, the nonwoven sheet becomes stiff without softness, so that a user cannot use the nonwoven sheet comfortably. In addition, Japanese Unexamined Patent Publication No. H5-279985 does not describe water disintegratability of the nonwoven sheet and does not teach how to provide excellent water disintegratability together with improved sheet strength.